The present invention relates generally to gas turbine engines, and more particularly to internally cooled airfoils used in such engines.
Gas turbine engines, such as aircraft jet engines, include many components (e.g., turbines, compressors, fans and the like) that utilize airfoils. Turbine airfoils, such as turbine blades and nozzle vanes, which are exposed to the highest operating temperatures, typically employ internal cooling to keep the airfoil temperatures within certain design limits. A turbine rotor blade, for example, has a shank portion that is attached to a rotating turbine rotor disk and an airfoil blade portion which is employed to extract useful work from the hot gases exiting the engine's combustor. The airfoil includes a blade root that is attached to the shank and a blade tip that is the free end of the airfoil blade. Typically, the airfoil of the turbine rotor blade is cooled by air (normally bled from the engine's compressor) passing through an internal circuit, with the air entering near the airfoil blade root and exiting near the airfoil blade tip as well as through film cooling holes near the airfoil blade's leading edge and through trailing edge cooling holes. Known turbine blade cooling circuits include a plurality of radially-oriented passageways that are series-connected to produce a serpentine flow path, thereby increasing cooling effectiveness by extending the length of the coolant flow path. It is also known to provide additional, unconnected passageways adjacent to the serpentine cooling circuit.
Turbine rotor blades with internal cooling circuits are typically manufactured using an investment casting process commonly referred to as the lost wax process. This process comprises enveloping a ceramic core defining the internal cooling circuit in wax shaped to the desired configuration of the turbine blade. The wax assembly is then repeatedly dipped into a liquid ceramic solution such that a hard ceramic shell is formed thereon. Next, the wax is melted out of the shell so that the remaining mold consists of the internal ceramic core, the external ceramic shell and the space therebetween, previously filled with wax. The empty space is then filled with molten metal. After the metal cools and solidifies, the external shell is broken and removed, exposing the metal that has taken the shape of the void created by the removal of the wax. The internal ceramic core is dissolved via a leaching process. The metal component now has the desired shape of the turbine blade with the internal cooling circuit.
In casting turbine blades with serpentine cooling circuits, the internal ceramic core is formed as a serpentine element having a number of long, thin branches. This presents the challenge of making the core sturdy enough to survive the pouring of the metal while maintaining the stringent requirements for positioning the core. Furthermore, the thin branches of the serpentine core can experience relative movement if not stabilized in some manner. Thus, core ties (i.e., small ceramic pins connecting various branches) are used to accurately position the core and prevent relative movement of the core branches such that the thicknesses of the walls separating adjacent passageways of the serpentine cooling circuit are controlled better. After casting, when they have been removed along with the core, the core ties leave holes in the walls. These core tie holes provide unwanted flow communication between adjacent passageways due to a pressure differential that typically exists between the two passageways. That is, cooling fluid in the higher pressure passageway will flow into the lower pressure passageway through the core tie hole. This will result in an undesirable cooling flow distribution compared to the original design intent.
Accordingly, there is a need for a turbine airfoil in which cooling fluid flow through core tie holes is minimized.